A temperature sensor element is designed to output electrical signals, such as currents, in response to a resistance change due to a temperature change in air, fluids such as water, or walls.
Among temperature sensor elements are thermistors with negative temperature coefficients (NTC).
With reference to FIG. 1, a structure of a conventional CIG (chip-in-glass)-type temperature sensor element is illustrated.
As shown in FIG. 1, the conventional temperature sensor element includes a temperature-sensing ceramic unit 10, first and second electrode 21 and 22 located, respectively, on and underneath the top and bottom of the temperature-sensing ceramic unit 10, lead lines 31 and 31 connected respectively to the first and second electrodes 21 and 22, and a vitreous protective unit 40.
The temperature-sensing ceramic unit 10 is formed by sintering a semiconductor ceramic material and changes in electrical resistance in response to the temperature sensed.
The first and second electrodes 21 and 22 are composed mainly of a material with high conductivity, such as silver (Ag), gold (Au), silver-palladium alloy (AgPd), platinum (Pt), etc., and each form an ohmic contact with the temperature-sensing ceramic unit 10 to enhance an electric contact between the temperature-sensing ceramic unit and the first and second lead lines 31 and 32.
Respectively connected with the first and second electrodes 21 and 22, the first and second lead lines 31 and 32 function as terminals that input and output electric signals into and from the first and second electrodes 21 and 22.
For electric connection to the first and second electrodes 21 and 22, the lead lines 31 and 32 are also made of a conductive material, such as nickel (Ni), copper (Cu), iron-nickel alloy (Fe—Ni), nickel-copper (Ni—Cu) alloy. As a lead line in conventional temperature sensor elements, dumet wire, which is a borax-coated copper clad wire with an iron-nickel alloy core, is used.
The vitreous protective unit 40 is made of transparent, insulating glass functioning to protect the temperature-sensing ceramic unit 10 and to immobilize the first and second lead lines 31 and 32 attached to the temperature-sensing ceramic unit 10.
Due to the vulnerability of the first and second leads 31 and 33 to oxidation, the temperature sensor element is allowed to measure a temperature of up to 500° C.
The temperature sensor element with the structure illustrated in FIG. 1 cannot measure a temperature higher than 500° C., such as 1000° C.
To fabricate a temperature sensor element capable of measuring a temperature higher than 500° C., a thermal treatment process at a high temperature is needed. In this regard, the first and second lead lines 31 and 32 should be made of a highly anti-oxidative material in order to prevent the surface oxidation thereof at such a high temperature. A platinum or platinum-based alloy is suitable for use in this purpose.
Because it is expensive, however, a platinum-based metallic material for the first and second lead lines 31 and 32 greatly increases the production cost of the temperature sensor element.